Starch consists of microscopic granules which swell in hot water or in cold or warm caustic solution. The granules do not swell in cold water or in very dilute caustic solution. Onset of swelling commences if temperature is increased beyond a critical point or caustic addition exceeds a critical concentration. This process is known as gelatinisation. The changes in the starch granules during gelatinisation are observable and progressive. Initially, there is a slight swelling of the granules and a loss of birefringence. A gradual observable increase in viscosity follows accompanied by further granular swelling until a plateau is reached. Only after the plateau stage do the granules become fully swollen to their maximum size. If the swelling process is not arrested, the granules continue to swell until they burst. The viscosity increases sharply followed sequentially by the folding of the granules and the appearance of exudate. After the exudate appears, the starch is no longer granular in form but is a stringy mass at which stage the starch is said to be fully or completely gelatinised. If the swelling is arrested in the plateau prior to maximum swelling or bursting of the granules, the starch is said to be partially gelatinised.
Starch based adhesives have been used for the manufacture of corrugated boards since the early 1930's. Basically the preparation processes fall into two groups:
(a) The CARRIER SYSTEMS, where the bulk of the adhesive is raw starch (approximately 80-85%) and the desired viscosity of the adhesive is derived from a proportion (approximately 15-20%) of the total raw starch, this proportion acting as a carrier after being fully gelatinised through heating and/or through treatment with strong alkali solution. The balance of the adhesive remains non-gelatinised starch. Various other lyophilic colloids have been used in place of fully gelatinised starch as a carrier. The liquid phase of such adhesives consequently is relatively viscous.
(b) The NO-CARRIER or CARRIER-FREE SYSTEMS are described in Australian specifications Nos. 290,991, 425,716 and 432,257, where the desired viscosity of the adhesive is obtained by swelling the starch granules in the adhesive by gentle alkali and/or heat treatment so that all of the starch is partly gelatinised. The liquid phase of such adhesives is very thin as, according to the respective patents, a negligible proportion of starch is fully gelatinised, i.e. in the form of lyophilic colloid.
During the last few years there has been a change in the concept of producing corrugated boards. This probably has been brought about by rapidly escalating heat-energy costs and the looming shortage of oil.
The generally tightening economic conditions and the consequent demands for higher production rates has resulted in innovation both in application techniques and in equipment design.
The current requirement is for production of corrugated board with the minimum evaporation of water from the glue-lines. This would have advantages not only in terms of savings in heat-energy consumption, but also in terms of better warp-control of combined boards.
Adhesive application equipment has been improved to enable uniform metering of adhesive at very low film thicknesses and it is now possible to apply less than half of the film thicknesses commonly used. To complement the better application equipment, hot-plate sections have been shortened and more adequately temperature-controlled. However, existing starch-based adhesive systems do not satisfy all the requirements of a modern corrugating plant where application of low film thicknesses of adhesive is highly desirable.
The necessity for an adhesive not only to have the correct rheological properties but also the correct water holding capacity for an individual machine is becoming more and more important. It is therefore desirable to have an adhesive preparation system that allows these important properties to be conveniently varied to enable "tailor-making" adhesives not only for each individual plant, but also for the different units within a plant.